Ever realized you needed to modify some dependency (e.g. I needed to add weak cache support to core.cache), go a head with forking on github, lein install your new version, modify your project.clj to pick it up, and then... somehow... you still seem to get the old version?
That's usually because some other dependency is explicitly requesting a version of that library. If you do lein classpath, you'll see your library show up, in two different versions, with the one you don't want first.